This research is dedicated to testing the hypothesis that all of the diagnostic information obtainable from a catheter-electrode recording from the heart's endocardial surface, near the bundle of His and the bundle branches, is also available by appropriate recording from the body surface. We will attempt to overcome the basic problem of signal-to-noise ratio and difficulty in the choice of surface electrode sites by a number of techniques. These will include utilizing, prior to receipt of the surface signals by the computer, significantly better low-noise preamplifiers. The search for the best electrode sites to sense the signal from the bundle of His, will be enhanced by a rapid search of the pre-averaged surface records which makes available data in 142 surface recorded PR intervals for selection of the most promising sites for further waveform analysis. After visual and temporal verification that activity is likely arising from the His-Purkinje system, we will apply already existing computer programs and transfer matrices to examine the three dipolar or vectorial leads and an additional 21 scalar leads of the multipolar series. Leads particularly sensitive to signals temporally assignable to His-Purkinje activity will be searched for. Parallel animal work will occur with the added advantage of the introduction of artificial bipolar signals in the region of the His bundle, the right bundle branch and the left bundle branch for both radiographic and analytic location and relationship to resulting surface signals on energization. Finally, a computational model of the sinoventricular conduction system will be expected to give insight into the interpretation of the observed data, to provide clues as to which surface lead arrays should prove most fruitful in both the direct recording and the multipolar analysis, and to suggest the possible direct contribution of the Purkinje activation to the early QRS complex.